The present invention relates to an optical recording and reproducing apparatus comprising a recording light source for emitting a recording light beam, a reproducing light source for emitting a reproducing light beam and an objective lens for focussing commonly the recording and reproducing light beams onto an optical record medium.
Such an optical recording and reproducing apparatus has been known and there also are many kinds of optical record media such as an optical disc having a thin layer of photoresist or metal and an opto-magnetic disc using a rotation of a polarization plane. In such an apparatus, when an information signal is recorded, the recording light source is modulated by the information signal to be recorded and at the same time the reproducing light beam having a constant intensity is made incident upon a guide track or a previously recorded information track. The reproducing light beam reflected by the record medium is made incident upon a light detector to derive focussing and tracking control signals. In order to position correctly spots of the recording and reproducing beams with respect to the tracks in the record medium, it is advantageous to focus these beams commonly by a single objective lens. To this end, these light beams must be composed by suitable optical members into a substantially single light beam. However, in practice, it is rather difficult to effect such a composition by means of usual optical members without causing a loss of light intensity. For instance, if a half mirror is used, half of the light intensity is lost. It should be noted that even if small semiconductor lasers are used as the recording and reproducing light sources, they cannot be arranged close enough to each other to produce laser beams which can be assumed to be substantially a single beam.
In general, since the output power of the recording light source is higher than the reproducing light source, an ambient temperature in the proximity of the recording light source becomes higher than that in the proximity of the reproducing light source. Therefore, the degree of thermal deformation of various optical members arranged near the recording and reproducing light sources might vary. This variation results in a deviation of a positional relation between the beam spots and the tracks on the record medium. For instance, a pitch of successive information tracks formed on the record medium when recording might be varied. Moreover, in such an event, the focussing and tracking control signals could not be obtained correctly.